LED lamps

ABSTRACT

A high power LED lamp has a GaN chip placed over an AlGaInP chip. A reflector is placed between the two chips. Each of the chips has trenches diverting light for output. The chip pair can be arranged to produce white light having a spectral distribution in the red to blue region that is close to that of daylight. Also, the chip pair can be used to provide an RGB lamp or a red-amber-green traffic lamp. The active regions of both chips can be less than 50 microns away from a heat sink.&lt;/PTEXT&gt;

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of application Ser. No09/549,575, filed Apr. 14, 2000, now abandoned which was acontinuation-in-part of application Ser. No. 09/196,358 filed Nov. 19,1998.

FIELD AND BACKGROUND OF THE INVENTION

The present invention s related to colored and white LED lamps. Apurpose of the present invention is to provide a single-chip LED lampthat is up to 250 times ore more powerful than a conventionalsingle-chip LED lamp. Another purpose of the invention is to provide atwo-chip white LED lamp that is up to 250 times more powerful than aconventional single-chip white LED lamp.

Existing single-chip LED lamps are generally of low input power,typically 40-150 milliwatts.

Designers of lamps for outdoor use have had the choice mainly ofincandescent tungsten light sources, gas discharge light sources, andLED light sources. Designers of LED-based lamps requiring high powerhave had to resort to using clusters of single chip LED lamps, each ofabout 0.1 watt rating, wired in series or in parallel and housedtogether as a single lamp unit to achieve enhanced power. Providing alamp unit by clustering single-chip LED lamps is very costly, since itinvolves making several single-chip lamps, housing them in a unit,wiring them up, and testing the final unit. U.S. Pat. Nos. 5,382,811 and5,632,551 provide examples of cluster lamps.

Applications for high power LED colored lamps include outdoor displays,which usually have to operate in direct sunlight and so require powerfullamps, and vehicle lights and traffic lights. At present long-life LEDtraffic lights for use at street intersections need more than a hundredconventional single-chip LED lamps for each lamp unit. The need to usemany LED lamps to provide a single LED traffic light is a disadvantage,particularly since only one tungsten light source is needed for aconventional traffic light.

Current blue-green LED traffic lights rely on gallium nitride (GaN) LEDtechnology; whereas amber and red traffic lights rely on aluminumgallium indium phosphide (AlGaInP) technology. It is an object of thisinvention to provide high power single chip LED lamps in both of thesetechnologies.

Applications for high power LED white lamps include vehicle headlightsand reverse lights, vehicle internal lights, torches and other batterypowered lighting devices. White single chip LED lamps are available, butthey are typically of only about 0.1 watt, unless they are clusterlamps. Furthermore, they rely on a GaN chip that generates ultravioletor blue light. All or most of this generated light energy has to beconverted, using fluorescent material, into longer wavelength componentsto produce the white light. The light conversion results in loss oflight energy. FIG. 31 illustrates the typical spectral distribution ofthe white light produced. This is quite different from the spectraldistribution of daylight, which is represented by dotted line 437.

Prior art single-chip LED lamps having clear convergent lenses, usedwidely in outdoor displays, suffer not only from the fact that they areof low power but also from the fact that they project light that is notuniform. The non-uniformity is partly due to the bonding pad or pads onthe chip top face, which are projected by the lamp as dark areas. Thetypical width of the bonding pad is about 30-40% of the width of thechip and this is large enough to interfere with achieving gooduniformity of projected light even if the LED lens is defocussedrelative to top face of the chip. For good quality image displays it isdesirable to match the apparent brightnesses of the viewed lamps towithin 5%. To achieve this it is important to reduce the non-uniformitycaused by the bonding pads.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a single-chip LED lampthat avoids or reduces the need for clustering.

A further object of the present invention is to provide a single chipLED lamp arranged so that light emitting portions of the chip areadjusted to be equalised in intensity.

A further object of the present invention is to provide a single chipLED lamp in which the sizes of the bonding pads relative to the size ofthe chip are reduced, so as to improve uniformity of light projected bya lensed lamp.

A further object of the present invention is to provide a single chipLED lamp, with input power in the region of 5-25 watts, that has lowrise of the junction temperature when energised, thus prolonging thelife of the lamp and reducing or eliminating the need for forcedventilation of the lamp.

A further object of the present invention is to provide a single chipLED lamp arranged so that light emitting portions of the chip that arefaulty, by drawing more than their fair share of current, are starved ofelectrical power.

A further object of the present invention is to provide a single chipAlGaInP lamp of high power, avoiding the need for clustering lamps foran amber or red street traffic light.

A further object of the present invention is to provide an AlGaInP LEDchip for an LED lamp that does not require a thick (and thereforecostly) window layer, either above or below the active region, forefficient light extraction.

A further object of the present invention is to provide a white lightlamp based on just two LEDs one of which has an AlGaInP active region.

A further object of the present invention is to provide a white lightlamp having a spectral distribution close to that of daylight.

A further object of the present invention is to provide a high power RGBlamp that is based on two chips one of which has an AlGaInP activeregion.

According to an aspect of the invention an LED lamp includes an LED chiphaving a top face and comprising a substrate and semiconductor layersbetween the top face and the substrate, the semiconductor layers formingthe core of a light guide extending parallel to the plane of the topface. The chip includes at least one cavity with light-emitting sidewalls that extends into at least one of the semiconductor layers. Thechip converts guided light in the core into top light. According toanother aspect of the invention guided light in the core is extractedwith the aid of reflectors that are parallel to the semiconductorlayers. According to yet another aspect of the invention metal tracksconnected to the n-type semiconductor layer are provided that enhancethe efficiency of the lamp.

According to another aspect of the invention an LED lamp includes a chipcomprising at least two light emitters each having a triangular topface, the two emitters being separated by a trench.

According to another aspect of the invention a single chip LED lampwhich can have input power of 5-25 watts includes an LED chip having atop face and comprising a substrate and semiconductor layers between thetop face and the substrate, the semiconductor layers forming the core ofa light guide extending parallel to the plane of the top face. The chipincludes at least one trench, and a heat sink is attached to the topface of the chip that draws heat from the active region of the chip.

According to another aspect of the invention an LED lamp has an LED chipwith a plurality of individually powered light emitting elements eachprovided with a fuse. This improves the yield of usable LED chips duringmanufacture.

According to another aspect of the invention there is provided an LEDlamp comprising: an LED semiconductor core having a thickness andcomprising a plurality of vertically stacked semiconductor layers;cavities in the core having side walls that divert light from the core;first and second reflectors that are below and above the core,respectively; each of the reflectors being reflective to light from thecore that has an angle of incidence to the reflector of 60 degrees, thereflectors guiding light generated in the core towards the cavities; andthe core being lattice-matched to GaAs and generating visible light.

According to another aspect of the invention a high power AlGaInP LEDfor a lamp is manufactured using the steps of: providing a member havinga planar surface; providing a GaAs substrate on which an AlGaInP LED isepitaxially grown; providing an electrical terminal on the LED; joiningthe member to the LED; removing the GaAs substrate; and the method alsoincluding the step of providing an opening in the member positioned tobe opposite said terminal. The steps are preferably carried out duringwafer processing.

According to another aspect of the invention a high power AlGaInP LEDfor a lamp is manufactured using the steps of: providing an LED that issupported on and epitaxial with a GaAs substrate; providing anon-semiconductor substrate having lower and upper surfaces; joining theLED to the upper substrate surface using a medium having a meltingtemperature exceeding 300° C.; removing the GaAs substrate; providing anohmic contact to the LED using a process that heats the LED to more than300° C. and providing a surface between the LED and the lower substratesurface that is reflective to LED light incident to the surface at 60°.The steps are preferably carried out during wafer processing.

According to another aspect of the invention there is provided an LEDwhite lamp comprising a vertical stack of just two LEDs one of which isof AlGaInP and the other of GaN (or InGaN). The AlGaInP LED generateslight at very low cost per lumen compared with GaN. Thus the combinationprovides white light at a lower cost per lumen than a white lamp basedentirely on GaN. Furthermore, the use of fluorescent material, whichcauses energy loss, can be avoided.

According to another aspect of the invention there is provided an LEDwhite lamp comprising an LED in which the active region crystal latticestructure is based on several chemical elements and the ratio of one ofthe elements to another of them is altered during the crystal growth soas to broaden the spectral distribution of the light of the LED.

According to another aspect of the invention there is provided a highpower variable color RGB lamp comprising a vertical LED stack of justtwo LED chips, one of which is of AlGaInP.

According to another aspect of the invention there is provided a lampcapable of generating white light, the lamp comprising a GaN chip havinginterleaved light emitting elements of differing colors.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates in side view an LED light source according to anembodiment of the invention.

FIG. 2 is a plan view of the LED chip in FIG. 1.

FIG. 3 is a sectional view of a portion of chip 2.

FIG. 4 is a cross-sectional view of a trench having a central ridge.

FIG. 5 illustrates a light source according to a further embodiment ofthe invention;

FIG. 6a illustrates light paths for an LED element having a rectangulartop light-emitting surface.

FIG. 6b illustrates light paths for an LED element having a triangulartop light-emitting surface.

FIGS. 7a, 7 b illustrate connection of a reflector track to a lowersemiconductor layer.

FIG. 8 illustrates a reflector cup for the chips of FIGS. 2, 5.

FIG. 9 illustrates an embodiment of the invention having a heat sink onthe chip top face.

FIG. 10 illustrates an alternative form of trench.

FIG. 11 illustrates an embodiment of the invention using an AlGaInPlight source.

FIG. 12 illustrates light extraction from an AlGaInP light source.

FIG. 13 illustrates another embodiment of the invention using AlGaInP.

FIG. 14 illustrates layers of the arrangement of FIG. 13.

FIG. 15 illustrate light rays in the layers of FIG. 14.

FIG. 16 illustrate manufacturing steps for producing the arrangement ofFIG. 13

FIG. 17 illustrate a method of producing an AlGaInP LED according to theinvention.

FIG. 18 illustrates a method of making electrical contact to asemiconductor layer.

FIG. 19 illustrate a further method of producing an AlGaInP LEDaccording to the invention.

FIG. 20a illustrates in plan view an embodiment for generating whitelight.

FIG. 20b is an elevation view of the arrangement of FIG. 20a.

FIG. 21 illustrates production of white light by the arrangement of FIG.20a.

FIG. 22 illustrates layers of the arrangement of FIG. 20a

FIG. 23 illustrates another embodiment for producing white light.

FIG. 24 illustrates components of light produced by the embodiment ofFIG. 23.

FIG. 25 illustrates the spectral distribution of the white lightproduced by the embodiment of FIG. 23.

FIG. 26 illustrates an LED chip according to a further embodiment of theinvention, having elements of three colors.

FIG. 27 illustrates the spectral distribution of white light produced byemploying the chip of FIG. 26.

FIG. 28 illustrates an arrangement for making connections to a two-chipwhite light source.

FIG. 29 illustrates a chip according to another embodiment of theinvention.

FIG. 30 illustrates the spectral distribution of white light producedproduced by an RGB LED lamp.

FIG. 31 illustrates the spectral distribution of the white lightproduced by a prior art GaN violet chip covered with fluorescentmaterial.

FIG. 32 illustrates an arrangement for making electrical connection to aGaN LED from the sapphire side.

DESCRIPTION OF PREFERRED EMBODIMENTS

FIGS. 1-3 illustrate an improved LED light source according to anembodiment of the invention. Light source 1 comprises an LED chip 2mounted on a metal support 3 which may be part of a reflective bowl thefloor of which acts as a heat sink. Over chip 2 there is a cover 4 oftransparent dielectric material. FIG. 3 is a sectional view taken alongT—T of a portion 40 of chip 2.

Chip 2 comprises a transparent base 5 over which is formed a layer 6 ofn-type semiconductor material. A layer 7 of p-type semiconductormaterial is formed over n-layer 6. Each of semiconductor layers 6, 7 canbe less than 5 microns thick. Semiconductor layers 6, 7 are referred tocollectively as semiconductor 8. Each of the basic n, p layers 6, 7 mayinclude supplementary layers, not shown, that enhance the performance ofthe chip, or that simplify its manufacture. For example, p-layer 7 mayinclude a thin light-passing gold layer on top of it to, in effect,enhance the electrical conductivity of the P-layer, and the n-layer mayinclude a low resistivity semiconductor layer for enhancing electricalconnection within the n-region of chip 2. Active region 10 may include athin layer of active region material that enhances light generation orthat determines the color of the light generated. The whole of the planeof the top of chip 2 up to side faces 25 constitutes a first majorsurface of chip 2. The lower surface 80 of substrate 5 constitutes asecond major surface of chip 2. A reflector 9 is placed on the lowersurface 80 of-substrate 5. Reflector 9 can be a mirror coating of silveror aluminum.

Transparent cover material 4 has a refractive index n_(c) that isgreater than 1.25, and preferably greater than 1.4. Furthermore, n_(c)is less than the refractive index n_(s) of semiconductor 8. The value ofn_(c) is preferably such that n_(s) is more than 20% higher, and morepreferably more than 30%, higher than the value of n_(c).

Transparent substrate 5 is chosen to be of a material having arefractive index n_(b) that is at least 15%, and preferably at least20%, less than the refractive index n_(s) of semiconductor 8. Thearrangement is such that semiconductor 8 together with media 5, 4, actsas a planar light guide, with the light being guided in semiconductorcore 8.

Electrically connected to n-layer 6, by contact of its lower surfacewith n-layer 6, is a bonding pad terminal 11. Electrically connected top-layer 7, by contact of its lower surface with p-layer 7, is a bondingpad terminal 12. Formed into top face 13 of chip 2, for example byetching, are elongate cavities or trenches 14. The cross-section T—T oftrench 14 is illustrated in FIG. 3. Trench 14 has a floor portion 15that is in the n-layer and that is parallel to chip top face 13. Joinedto floor 15, and thereby electrically connected to the buried n-layer 6,is a metallic conductor track 16, comprising gold or other lowresistivity metal. For chips generating green or blue light the track 16preferably comprises metal of good reflectivity to green and blue light,such as aluminum.

Trench 14 is filled with transparent dielectric material 17 having arefractive index n_(t) that is less than 80% of the refractive index ofsemiconductor 8. Material 17 is preferably of plastic, such as epoxy.Material 17 may be the same as translucent material 4, or it may bedifferent. All conductors 16 are metallically connected to each otherand to bonding pad terminal 11. Thus current from a point 18 in theburied n-layer can reach n-terminal 11 via conductors 16 that are nearpoint 18, instead of only by passing through the whole length of then-layer between point 18 and terminal 11.

By means of conductors 16, the voltage drop between bonding pad 11 andany point on the n-layer, such as 18, is reduced.

This increases the lumens-per-watt efficiency of the chip. Furthermore,the distribution of current and hence the generated light throughout thechip top is made more even.

Conductors 16 comprise meandering tracks. Thus, for example, conductors16 a, 16 b together constitute a meandering track, i.e. a track thatchanges its direction. Conductors 16 c, 16 d constitute anothermeandering track. Conductor tracks 16 have node points 50 that join aplurality of tracks together. The width of conductor 16 can be 5-20 μmand its thickness can be 0.5 μm or more. Conductors 16 can be formed bythe same process steps as are used for making pad 11, or by similarprocess steps.

Joined to the top of p-layer 7, and thereby electrically connected top-layer 7, are electrical conductor tracks 19 shown only in FIG. 2,which may comprise gold or other metal. All conductors 19 aremetallically connected to each other and to bonding pad terminal 12.Thus current applied to bonding pad 12 can reach a point 21 in p-layer 7that is remote from bonding pad 12, mainly via conductors 19, instead ofvia the player only. Conductor tracks 19 have node points 22 that join aplurality of tracks together. Conductors 19 can be formed by the sameprocess steps as are used for making pad 12, or by similar processsteps. The network of conductors 19 comprises meandering tracks.

By means of conductors 19, the voltage drop between bonding pad 12 andany point on the player, such as 21, is reduced This increases thelumens-per-watt efficiency of the chip. Furthermore, the distribution ofcurrent and hence the generated light throughout the chip top is mademore even.

Conductors 19 may be 1-50 μm wide, or more, and 0.2 to 2 μm thick.Because of their narrow width, dark lines produced by conductors 19disappear when an image of light source 2 is projected by a defocussedlens. If the p-layer is such that it has a thin light-passing gold layerall over it, then, by incorporating conductors 19 to feed the p-layer,the thickness of the gold layer can be reduced, allowing more light topass through the gold layer and, therefore, increasing the top light ofthe chip, while still maintaining low resistance connection to thep-layer.

A ray of light generated within the chip at a point 23 can have any oneof many directions. If the direction of the ray is normal or nearlynormal to top face 13 of the chip it passes out of the chip as top light24. If the ray is parallel or nearly parallel to top face 13 of the chipit travels in semiconductor 8, which acts as a light guide core, untilit meets a side exit face. In the arrangement of FIGS. 1-3, there arefour outer side faces 25 of chip 2 and several supplementary side faces26 provided by trenches 14. Supplementary side faces 26 allow lightprogressing sideways in semiconductor layers 6, 7 to escape from thechip without having to travel all the way to outer side faces 25.Because the sideways light trapped between media 4, 5 travels a shorterdistance within semiconductor 8 before it escapes, it is attenuatedless. This lower light attenuation facilitates making large, bright LEDchips. Conductors 16, 19 result in lower electrical losses and so, also,facilitate making large, bright LED chips.

Light emanating from chip 2 is illustrated by various rays shown in FIG.3. Each trench wall 26 forms an obtuse angle 27 with the part of topface 13 that adjoins the side wall and emits light. Obtuse angle 27 ispreferably between 95 and 130 degrees, and more preferably between 110and 120 degrees. Wall 26 may have a curved cross section. In this caseangle 27 is measured between the plane tangential to the wall half wayup the height of the wall and top face 13.

The depth of trench 14 is preferably more than 50% of the thickness ofsemiconductor 8, so as to allow a large proportion of the light insemiconductor 8 approaching the trench to escape through trench wall 26.The thickness of the layer of n-type material under metal track 16 ispreferably greater than 5% of the total thickness of the n-layer, so asto maintain good electrical connection between track 16 and the body ofn-type semiconductor material either side of trench 14.

The paths of rays 28, 29 generated at the semiconductor active region 10are illustrated in FIG. 3. Because the index of refraction n_(s) ofsemiconductor 8 is higher than the index of refraction n_(t) of covermaterial 4 and trench wall 26 forms an obtuse angle with chip top face13, horizontal ray 28 hitting trench wall 26 is diverted downwards byrefraction as it passes through wall 26. Ray 28 is then reflectedupwards by reflector 16 to pass through aperture 33 of the trench. It isnot only horizontal rays that will pass through trench wall 26. Raysthat have been guided by multiple internal reflection in semiconductor 8that hit wall 26 with a small angle of incidence will also pass throughthe wall, such as ray 31.

Ray 29 hitting interface 30 between semiconductor 8 and base 5 at an anangle of incidence Ø_(i) of 45 degrees is totally reflected at interface30, as shown, and then again at interface 13. Ray 29 is propagatedsideways by the previously mentioned light-guide property of layers 4,8, 5. Total reflection of ray 29 at interface 30 occurs because theangle of incidence Ø_(i) is greater than the critical angle Ø_(csb) fortotal internal reflection at interface 30. The reflected ray 29 hits topsurface 13 at 45 degrees, and it is again totally reflected, because theangle of incidence of ray 29 to top surface 13 is greater than thecritical angle Ø_(csc) for the interface between semiconductor 8 andcover material 4. Ray 29 then hits trench wall 26. At trench wall 26 ray29 is yet again totally reflected. Total reflection of ray 29 occurs atsurface 26 because its angle of incidence is greater than the criticalangle Ø_(cst) for the interface between semiconductor 8 and trenchmaterial 17. After being diverted by reflection by trench wall 26, ray29 exits semiconductor 8 through interface 30. Ray 29 exitssemiconductor 8 because it hits interface 30 at an angle of incidencethat is less than the critical angle Ø_(csb) for interface 30. Ray 29 isthen reflected by reflector 9 upwards to exit through chip top face 13as top light ray 29T.

Taking the case, for example, where:

Cover medium 4 and trench material 17 are both of epoxy having arefractive index of 1.5, i.e. n_(c)=n_(t)=1.5;

Base 5 is sapphire having a refractive index of about 1.7, i.e.n_(b)=1.7;

Semiconductor 8 has a refractive index n_(s)=3

Obtuse angle 27 has a value └₂₇=100°

Then:

Ø_(csc)=sin⁻¹ n _(c) /n _(s)=sin⁻¹0.5=30°

Ø_(cst)=sin⁻¹ n _(t) /n _(s)=sin⁻¹0.5=30°

Ø_(csb)=sin⁻¹ n _(b) /n _(s)=sin⁻¹1.7/3=sin⁻¹0.567=34.5°.

After it is diverted by face 26, the angle of incidence of ray 29 tointerface 30 is equal to: 45°−2(Ø₂₇−90°)=25°, which is less thanØ_(csb). If Ø₂₇ is increased to 110° the angle of incidence of ray 29 tointerface 30 reduces to 5°.

If the layers of semiconductor 8 have unequal refractive indexes, thevalue of n_(s) is taken to be equal to the highest of these unequalrefractive indexes. Substrate 5 may comprise a transparent supportcovered with a thin layer, next to core 8, having the refractive indexn_(b). In this case the transparent support may be of material, such assemiconductor, having a higher refractive index higher than n_(b). Forthe thin layer to provide good reflection to guided light the thicknessof the thin layer should be greater than the wavelength of the lightgenerated, and preferably greater than twice the wavelength of the lightgenerated.

Until it hits side wall 26, ray 29 is guided light travelling in thecore of a light guide. The guided light 29 becomes converted into toplight 29T. Light 29T is light that has passed through the p-layer 7 andtop face 13. The top surface of metal support 3 can be relied on forreflecting light at the bottom of substrate 5 if there is no reflectivecoating 9 on substrate 5.

The arrangement of FIG. 2 can be regarded as consisting of nineelemental areas E1-E9 each emitting top light, each having at least onecorner next to two trenches 14, and each covered with at least one topconductor 19. There is a trench 14 running between each adjacent pair ofareas E. Top light 24 from top-face 13 of each adjacent pair of elementsE is accompanied with trench light emanating from the trench between thepair. The chip can be large, for example up to 1000 μm or more wide, andit may have more than nine elements E. Pads 11, 12 can each be about 150μm square or less. The width of each element E is arranged to be severaltimes greater than width K of trench 14. For example, the width ofelement E5 may be 300 μm and K may be 15 μm. Trench width K ispreferably several times greater than the wavelength of the light fromthe active region, and is preferably more than twice the depth of trench14. Conductor 16 between a pair of elements E provides a low resistanceconnection between the pair.

Transparent dielectric material 17 filling the trench may have arefractive index n_(t) between 1.25 and 2.5. The value of n_(t) affectstransmission of light through wall 26. The higher the value of n_(t) thelarger the critical angle Ø_(cst) at the inner face of trench wall 26will be. With a larger critical angle more light can pass through trenchwall 26. Trench 14 may have a central ridge 32, illustrated in FIG. 4,which extends along its length. Track 16 covers ridge 32. Ray 28 isshown doubly reflected by ridged track 16.

FIG. 5 illustrates a further embodiment of the invention. Thearrangement differs from that of FIG. 2 in that it uses triangular lightemitting elements instead of rectangular ones, and in that it includestwo sets of light emitting elements connected in series. Chip 36 has 16triangular light emitting elements, TE1-TE16. Elements TE1-TE8 areconnected in parallel with each other. Also, elements TE9-TE16 areconnected in parallel with each other. There is insulation, not shown,under conductors 19 at the areas where they cross over trenches 14, andthere is insulation, not shown, under the whole of conductor 20. Then-layer for elements T1-T8 is isolated from the n-layer of elementsTE9-TE16. The isolation can be achieved by making trenches 114, betweenset TE1-TE8 and set TE9-TE16, sufficiently deep so that their floors 15are in substrate 5, instead of in n-layer 6. Trenches 114 may includeconductors 16, as shown, or not.

Current applied to p-terminal 12 is fed to the p-layers of elementsT9-T16 by means of conductors 19, and the current from conductors 16 ofelements TE9-TE16 passes through conductor. 20 to conductors 19 incontact with the p-layers of elements T1-T8. The current from conductors16 of elements TE1-TE8 passes into n-terminal 11. Thus chip 36 comprisesa first set of light generating elements (TE9-TE16 connected inparallel) connected in series with a second set of light generatingelements (TE1-TE8 connected in parallel). Light extraction from thetrenches can be as described before in relation to FIGS. 3, 4.

The use of triangular elements instead of rectangular ones enhanceslight extraction further. This is explained as follows. FIG. 6aillustrates in plan view an LED element embedded in epoxy 37 and havinga conventional (rectangular) top light-emitting surface. Epoxy 37 istaken to have a refractive index that is half that of the semiconductorof the element, resulting in the critical angle being 30 degrees. A ray38 that starts at a point 39 and and that is parallel to top face of theelement is shown hitting a side wall of the element at an incident angleØ_(i)=45°. Ray 38 is subjected to successive total internal reflectionand cannot escape from the element, as illustrated. By comparison withFIG. 6a, FIG. 6b shows in plan view an LED element embedded in epoxy 37and having a triangular top, such as the ones in chip 36. The elementhas side walls 41, 42, 43. Side wall 41 is inclined to sidewall 42 by anacute angle of 45 degrees. Also, side wall 43 is inclined to sidewall 41by an acute angle of 45 degrees. A ray 44 starting at point 39 canescape even if though it hits side wall 41 at 45 degrees, because afterinternal reflection it hits side wall 42 with an incident angle of zero.Rays 45, 46 also hit side walls at 45 degrees and subsequently escape.Thus the element with the triangular top is more efficient for lightextraction from the semiconductor than the element with a rectangulartop.

In the arrangement of FIG. 5, each of the elements TE may be providedwith an individually chosen adjuster A. Only two of the adjusters, A3,A5, are shown. Each adjuster A is in contact with the p-layer under it.The areas of the adjusters are chosen so that each element TE has thesame illumination as the other elements TE regardless of its position onthe chip. Thus, for example, adjuster A5 is larger than adjuster A3.Thus the series resistance between track 19 and the p-layer is less forelement TE5 than it is for TE3. By appropriate choice of the differentadjuster sizes, all elements TE can produce the same light even if thevoltages applied to them are not exactly the same, and even though someof the elements have only two associated trench conductors 16 instead ofthree. As an alternative to adjustment by elements A, the width of track19 can be varied from element to element to achieve the matching.Matching by differing elements A or by variation of the widths ofconductors 19 can also be provided for the elements E in the arrangementof FIG. 2. Uniformity of chip lighting of better than ±3% is desirablefor lensed lamps used in video displays, so as to provide uniformity ofthe light projected by each lamp lens.

Manufacture of a light source unit based on the chip of FIG. 2 or FIG.5, starting with a substrate wafer common to a plurality of final lightsource units, includes the steps, in order, of: applying a reflectivelayer, for reflector 9, to the bottom surface of the base wafer; formingthe semiconductor layers over the top of the wafer; forming trenches 14(and 114 in the case of FIG. 5) into the semiconductor by chemicaletching or by other means; forming tracks 16 in the trenches; insulatingtrenches 14 at least at the parts thereof where tracks 19 will crossover, and providing insulation for conductor 20 in the case of FIG. 5;forming tracks 19; dicing the wafer to provide separate chips; andfilling the trenches and covering the top and outer sides of each chipwith transparent dielectric material having a refractive index of morethan 1.25.

In the various arrangements discussed herein trenches 14 can extend intobase 5 so that interface 30 is above the top of the portion of track 16that is in contact with floor 15 of the trench. In this case, track 16can have contact tabs 16 t on both of its sides that are connected tothe two n-layer zones separated by the trench. FIGS. 7a, 7 b illustratea contact tab which emanates from track 16 to make electrical contactwith a small plateau 6 p in n-layer 6. Each of the elements E or TE canhave several contact tabs 16 t electrically connected to the element.

In the various arrangements discussed herein reflector 16 can beextended in width so that it covers part or all of each of its sidewalls 26. In this case the trench is arranged to have translucentdielectric material over at least a portion of each side wall 26 toprevent short circuiting the p-layer to the n-layer. Furthermore, in thevarious arrangements discussed herein trench 14 may have a side-wallprofile that includes one or more steps.

FIG. 8 illustrates another embodiment of the invention. Chip 51, of thetype of FIG. 2 or FIG. 5, mounted in a metallic reflective cup 52 andembedded in epoxy 4. The top of epoxy 4 (not shown) is formed as a lens.Cup 52 is made deep so that a ray 53 emanating from the central area ofthe top face of chip 51 at an angle w relative to the chip top face isdeflected by the cup. Angle w is greater than 30 degrees, and preferablygreater than 40 or 45 degrees.

FIG. 9 illustrates schematically a further embodiment of the invention.Light source 60 comprises a chip 61 which is preferably of the same orsimilar construction as that previously discussed for FIG. 2 or FIG. 5,but with no reflector 9 on substrate 5. Chip 61 may be as large as 5 mmwide and have many elements and have an input power of 5-25 watts. Forthe arrangement of FIG. 9, track 16 is preferably either of small width(preferably less than 0.5 K}, or of large width, about equal to K, asshown in FIG. 9. A transparent insulator 65 covers each of the twosemiconductor side walls. Track 16 covers each of insulators 65. Thusthe whole of the side wall 76 includes a metallic reflector insulatedfrom p-layer 7 and active region 10. The two side walls 76 each form anobtuse angle with top face 13. A heat sink 62 of aluminum, which has ahigh thermal conductivity and high optical reflectivity, is placed overchip 61. Heat sink 62 has integral spacer portions 64 drawing heat awayfrom chip top face 13. Heat sink 62 can be 5 mm thick, or more, and isthermally connected to a larger heat sink, not shown, that cools heatsink 62. Chip p-terminal 12 is connected by bonding wire 65 to aluminumheat sink 62 and hence to the p-terminal 66 of the lamp. Chip n-terminal11 is connected by bonding wire 67 to lamp n-terminal 68 which issupported on insulator 69. Openings 63 a, 63 b in heat sink 62 allowaccess to the chip terminals to achieve bonded wiring 65, 67. Afterbonding of wires 65, 67 epoxy is injected into openings 63 a, 63 b toprotect wires 65, 67 and to cover top face 13 with epoxy. Covering topface 13 with epoxy provides perfect reflection for guided light andenhances extraction of top light from chip 61. After leaving the chipthe top light is reflected by the under surface of heat sink 62 to passback into the chip and out of substrate 5. Faces 77 of spacers 64 can besmall, so that most of top face 13 is covered with epoxy, and hence mostof the reflections of guided light by interface 13 are 100% totalinternal reflections. For the internal reflections occurring at faces77, a proportion of the light energy is lost at each reflection. Spacers64 can be numerous, so that they provide adequate cooling of thesemiconductor junction 10. Thermal connection between spacers 64 andchip top face 13 can be enhanced by coating the lower faces of spacers64 with a suitable heat transfer compound before pressing heat sink 62onto chip 61. Epoxy 70 may be laid on top face 13, for example by padprinting, prior to pressing heat sink 62 onto the chip and as a separateoperation from filling cavities 63 a, 63 b. Any translucent materialhaving a refractive index greater than 1.25 may be used instead of epoxyfor medium 70. The thickness of medium 70 under heat sink 62 ispreferably greater than the wavelength of the light of the chip. Thethickness can be several times the wavelength. The lower surface of heatsink 62 may be covered with an electrically insulating coating.

A 25 watt chip can have 256 light emitting elements E or TE. It is notessential to have perfect yield of the light emitting elements. Ifproduction quality is such that a few of the elements do not emit light,the lamp will still function to give powerful light.

Referring to FIG. 2, each of elements E2, E4, E5, E6. E7, E9 has anindividual p-conductor 19 energising the element. It is possible bymodification of the wiring to arrange that all the elements E haveindividual p-conductors. In this case each individual p-conductor mayhave a fuse, such as fuse 71 illustrated for element E6, arranged toburn out when the current to the element exceeds the intended currentfor the element by a factor of, for example, 2.5. By this means,short-circuited elements in the chip are automatically isolated when thepower is applied to the chip. The triangulated chip of FIG. 5 also mayhave its wiring re-arranged to provide individual fusing of each of theelements TE. Fuse links 71 of elements E, TE that draw excessive currentmay be burnt out, i.e., disrupted, at production time to render themopen circuit by applying a test current to the chip. Alternatively, fuselinks 71 of elements E, TE that draw excessive current may be may bedisrupted by a laser beam directed at them. Determination of which ofthe fuse links 71 are to be disrupted with the laser beam can be carriedout with the aid of a heat-sensing camera directed at the chip while thechip is energized. In this case elements E or TE that are, for example,20 degrees hotter than the average temperature of all the elements areidentified as the ones drawing more than their allowable current and areselected for disruption of their links 71 using the laser.

A lamp of 256 elements can be wired as 16 blocks in series, each blockhaving 16 elements TE connected in parallel, with each element TE havingits own fuse. With this arrangement failure of an element TE, duringmanufacture or during service, does not put another element TE out ofaction. If, for example, in each bock four random elements TE are opencircuit, the lamp will still function. With the yield of good elementsTE at only 75% the lamp gives about 75% of the light of a lamp having nofaulty elements TE.

FIG. 10 shows another arrangement for trench 14 that can be used in anyof the chips described herein. As shown, trench 14 cuts into substrate5, and includes translucent dielectric 65 over the p-layer portion ofstepped side wall 26. Track 16 makes electrical contact with n-layer 6at side wall 26.

In the various arrangements that have been discussed herein, layers 6, 7and the active region 10 may each comprise nitride of two or more ofgallium, indium and aluminium, or nitride of just one of these elements,and the substrate can be sapphire.

FIG. 11 illustrates schematically another embodiment of the invention.Semiconductor core 8 comprises LED layers with the active region 10generating amber or red light, such as may be required for an amber orred single-chip traffic light. The construction of FIG. 11 is providedby starting with the construction of FIG. 9 using in the construction ofFIG. 9 an LED chip 61 having a core 8 comprising AlGaInP LED layerslattice matched and joined to a substrate 5 of GaAs. Core 8 may includea window layer of p-type GaP above the AlGaInP LED layers. Although forsimplicity of illustration only one light extraction trench 14 is shown,there are in fact several light extraction trenches, the arrangement ofcore 8 being preferably the same as or similar to the arrangements ofFIG. 2 or FIG. 5. Member 62 is the substrate for core 8 during and afterremoval of the GaAs. Removal of the GaAs substrate completes themanufacture of semiconductor core 8. Member 62 can be of the same widthas core 8 or wider. The lower surface of heat sink 62 is madeelectrically non-conductive to avoid unwanted electrical contact betweenthe heat sink and the top surface of core 8.

After the attachment of the AlGaInP LED chip 61 to heat sink 62 as shownin FIG. 9, the GaAs substrate 5 of chip 61 is selectively etched off,leaving the lower surface of the AlGaInP LED core 8 exposed. GaAs isabsorbent to visible light. Selective etching of the GaAs substrate 5may be achieved using, for example, a mixture of hydrogen peroxide andammonium hydroxide. Core 8 may include as its lowest layer asemiconductor stop-etch layer that enhances the resistance of core 8 tothe etchant. As a protection against unwanted etching sideways of core8, the four outer side surfaces of LED core 8 can be covered withmaterial that is insoluble by the etchant. Heat sink 62, also, mayoptionally be coated for protection against the etchant. Heat sink 62provides structural support for core 8 during removal of the GaAs andafter this removal. After chip substrate 5 has been removed, the lowersurface of LED core 8 is covered with epoxy 70, which can be formed as alens. For enhanced protection of core 8 against strain and againstingress of moisture, a sheet of glass having a refractive index of1.5-1.8 may be bonded to the lower surface 30 of core 8, covering all ofthat surface.

The AlGaInP LED lamp of the invention is not limited to having a lightextraction cavities 14 of the form shown in FIGS. 9, 11. The the lightextraction cavities may take any of the other forms previously describedin relation to the chip patterns of FIGS. 2, 5.

FIG. 12 illustrates one alternative cavity arrangement 14 that can beused instead of the cavity arrangement 14 shown in FIG. 11. The cavityof FIG. 12 is similar to that shown in FIG. 3. It is seen from FIG. 12that light reflected upwards in cavity 14 by reflector 16 is reflecteddownwards by the reflective surface of heat sink 62 to pass through core8. Thus the light reflected upward by reflector 16 can escape from thelamp.

AlGaInP core 8 has a refractive index of about 3.3 and epoxy 70 in FIG.12 may have a refractive index of about 1.5. In this case core 8 will bea light guide to all light generated by active region 10 that has anangle of incidence to surface 13 or surface 30 exceeding about:

sin⁻¹(1.5/3.3)=27°

Thus most of the light generated by active region 10 of the AlGaInP LEDwill be trapped in core 8 until it reaches one of the light extractioncavities 14. On reaching the various light extraction cavities 14 animportant proportion of the trapped light will escape from core 8. FIG.12 illustrates the entrapment and escape of ray 29 having an initialangle of incidence to core face 30 of 45°.

FIG. 13 illustrates schematically a further embodiment of the invention.Light source 90 includes a heat sink 86 which may be of metal. The topof heat sink 86 is made reflective. Above heat sink 86 is a reflectinglayer 87. Layer 87 is of translucent amorphous material, such as epoxy;i.e., layer 87 is not a crystal. Above layer 87 is a semiconductorlight-guide core 8 comprising layers of AlGaInP. Formed into core 8 is aset of light-extraction cavities 14 and conductors 16, 19, for examplehaving a plan view shown in FIG. 2 or FIG. 5. Above core 8 is a glasssheet 81 having a through-hole 82 opposite each of chip terminals 11,12. Glass sheet 81 may have a thickness of between 0.2 and 1 millimetersand a refractive index of between 1.5 and 1.8. Glass sheet 81 hasattached thereon a pair of metallic contact pads 83, 84 which areelectrically joined to chip terminals 12, 11 respectively by bondingwires. Glass sheet 81 is covered with a lens 89 formed from translucentepoxy. Layer 97 of epoxy bonds glass sheet 81 to core 8.

The device of FIG. 13 has no semiconductor substrate. Amorphous sheet 81serves as a substrate during manufacture of the device; and reflectiveamorphous sheet 86 serves as a final substrate for mounting the deviceonto a heat sink.

FIG. 14 illustrates an element E or TE and a preferred arrangement forthe semiconductor layers of core 8 of FIG. 13. Referring to FIG. 14,layer 7 comprises an AlGaInP p-layer 7 a next to active layer 10. Activelayer 10 is preferably of AlGaInP. A GaP layer 7 b of higher electricalconductivity than layer 7 a is placed over layer 7 a. Metal n-track 16has a raised portion 116 that reduces the longitudinal resistance of thetrack. Layer 6 consists of an AlGaInP layer 6 a next to active layer 10and an AlGaInP layer 6 b under layer 6 a. Layer 6 b is of higherelectrical conductivity than layer 6 a and acts as a translucent contactlayer for energising layer 6 a. Metal n-track 16 has a raised portion116 that reduces the longitudinal resistance of the track. Cavities 14are filled with translucent material 88 having a refractive index ofless than 2.5, for example epoxy. Holes 82 are filled with material 85through which moisture will not pass.

Heat sink 80 may be provided at its top with optional spacers 64. Heatsink 86 may be joined at its lower face to another, larger heat sink,not shown. Glass sheet 81 should have a thermal coefficient of expansionless than 20×10⁻⁶K⁻¹. For minimal strain of semiconductor core 8, glasssheet 81 should have about the same thermal coefficient of expansion assemiconductor core 8.

Both of layers 87, 97 may be of epoxy, in which case each of the layers87, 97 will be reflective to all rays having an incident angle Ø_(i)that is greater than about sin⁻¹1.5/3.3=27° (3.3 being an approximatevalue for the refractive index of GaP, as well as AlGaInP). Thus each oflayers 87, 97 will be reflective to rays incident to it from core 8 at45°.

FIGS. 15a, 15 b, 15 c illustrate escape of light guided in core 8. It isseen that each of rays 101. 102. 103 is reflected by both lowerreflector 87 and upper reflector 97 before escaping from core 8. Rays102, 103 are incident to reflectors 87, 97 at 45°. Ray 101 is incidentto reflectors 87, 97 at about 55°.

The width of trench 14 is greater at the open top of the trench than itis in the bottom of the trench, allowing light passing through sidewalls 26 a, 26 b to escape easily from the trench. As an alternative tousing a translucent medium for layer 87, layer 87 may be a metallicreflector, in which case spacers 64 are not used. As anotheralternative, layer 87 may be eliminated, and the reflective top of heatsink 86 relied upon to act as a reflector. Since glass has about thesame refractive index as epoxy, layer 97 may be eliminated.

Manufacture of the AlGaInP lamp of FIG. 13 can be achieved using, forexample, the following steps. Starting with a GaAs wafer, the AlGaInPlayers 6 b, 6 a, 10, 7 a, and a GaP layer 7 b are grown over the GaAssubstrate, providing the core 8, as represented by FIG. 16a. Because theAlGaInP layers are grown on GaAs they have the same lattice constant asGaAs. From the wafer of FIG. 16a a plurality of devices are formed, eachdevice having, for example, all the features shown in FIG. 2, or all thefeatures shown in FIG. 5, i.e. including trenches 14 and conductors 16,19, 11, 12 and optional fuses 71.

Stepped trenches 14 can be formed in the wafer of FIG. 16a by etching intwo stages as illustrated by FIGS. 16b, 16 c. The first etching stage iscarried out using a process or etchant that is selective to removingGaP. Such an etchant will expose the top of layer 7 a. The next etchingstage is carried out using a process or etchant that removes the AlGaInPlayers down to floor 15 in layer 6 b. An advantage of etching in twostages is that it can give more precise depth of etching into theAlGaInP layers, since the etching of the AlGaInP layers starts exactlyat the top of layer 7 b, which is closer to trench floor 15 than is thetop of layer 7 b. The combined thickness of layers 6 b, 6 a, 10 and 7 acan be less than three microns and the thickness of layer 7 b can exceedfifteen microns.

Next, the n-tracks 16 are formed, as represented by FIG. 16d. At thesame time as forming tracks 16, connection pads 11, 12 are formed. Next,the p-tracks 19 are formed (not shown in FIG. 16). Next, cavities 14 arefilled with epoxy, or with other translucent material of refractiveindex less than 2.5. Next a perforated glass sheet 81 large enough tocover all the devices on the wafer is bonded onto the top of layer 7 ofthe wafer, with the sets of perforations 82 positioned to coincide withthe sets of bonding pads 11, 12 as shown in FIG. 13. The bonding of theglass sheet to the top of the wafer can be done using epoxy or by othermeans. Sets of terminal pads 83, 84 are provided on the glass sheet, forexample prior to bonding the glass sheet to the wafer. FIG. 16eillustrates glass layer 81 attached to the wafer.

Next, for each of the devices formed on the wafer, terminal 12 is joinedto terminal pad 83 and terminal 11 is joined to terminal pad 84, usingthermocompression bonding as illustrated in FIG. 13, or by other means.Next, holes 82 are filled with moisture-stopping filler 85. Next, theGaAs layer is removed, as represented by FIG. 16f.

Next, a heat sink sheet 86 having a reflective top face is bonded to thewafer, using epoxy for example, as represented by layer 87 in FIG. 16g.As a final stage of wafer processing, the wafer is divided into separatedevices, each having a pair of terminals 83, 84 by which the device canbe energized.

Glass sheet 81 provides a substrate that supports core 8 during removalof the GaAs. If desired, the step of joining a heat sink layer 86 to thewafer can be eliminated; in this case the wafer is divided into theseparate devices and then each device is joined to a heat sinkindividually.

Semiconductor layer 6 b may be of GaP instead of AlGaInP, or it maycomprise a layer of AlGaInP over a layer of Gap. Furthermore, conductortracks that are in ohmic contact with the lower surface of layer 6 b maybe provided to supplement or replace the electrical function of tracks16. GaP can be grown on the underside of layer 6 b in FIG. 16f providedtransparent body 81 and filler 88 are selected to withstand thetemperature needed for growing the Gap.

Another manufacturing procedure for achieving high light output isillustrated in FIG. 17. On a gallium arsenide substrate 105 is grown ann-type AlGaInP layer 107 a, an AlGaInP active layer 110, a p-typeAlGaInP layer 106 a and a p-type GaP layer 106 b, as represented by FIG.17a. Next, as shown in FIG. 17b, a metal heat sink layer 111 having areflective top face is bonded to layer 106 b using a translucent bondinglayer 112 of material that can withstand and remain solid at the hightemperatures used in subsequent wafer processing, for example 300-500°C. for contact alloying and 500-700° C. for growth of GaP layer 106 b.Layer 112 may be a printed layer of glass powder, or other material ofhigh melting temperature, that is melted to achieve the bonding. Next,GaAs layer is removed as represented by FIG. 17c. Next, an n-type GaPlayer 107 b is grown over layer 107 a, as represented by FIG. 17d.

Next, trenches 14 are formed. Next trench conductors 16 in contact withp-layer 106 b are formed. A p-terminal, not shown in FIG. 17e, is alsoformed in contact with p-layer 106 b. Next, top conductors 119 areformed in contact with n-layer 107 b and an n-terminal, not shown, alsois formed in contact with n-layer 107 b. The conductors and terminals ofcore 8 can be formed by known methods involving deposition and etchingof doped metal; followed by heat treatment at a temperature above 300°C. (typically 400-500° C.) to cause the metal dopants to migrate intothe semiconductor and thus provide good ohmic contact. Translucentbonding layer 112 must withstand the heat treatment temperature andshould not be molten at that temperature. The wafer is finally dividedinto separate lamp-chips each of which may be five millimeters wide. Foreach lamp-chip, heat sink 111 is joined to a larger heat sink. Sincelayer 111 serves as a substrate for the chips it is made substantiallythicker than core 8, for example 100-1000 microns thick.

FIG. 18 illustrates a further embodiment of the invention, usingelectrical connections made to the lower surface of core 8 of an AlGaInPlamp. Optional track 115 ohmically connected to layer 106 b cansupplement or replace the electrical function of trench track 16.Conductor pad 117 ohmically connected to layer 106 b can serve as one ofthe terminals of the chip, and it can be electrically connected to heatsink 111 by solder 118 as shown, or by other means. Ohmic contacts 115,117 to layer 106 b can be provided, for example, just after formation ofthe structure of FIG. 17a.

Substrates 86, and 111 of FIG. 17, can be of translucent material, suchas glass. In this case metal reflector track 16 can be extended in widthso as to cover the trench side walls, provided track 16 is electricallyinsulated from the side walls. Substrate 111 and bonding layer 112 canbe one translucent item deposited on layer 106 b, for example aglass-like sheet that is placed onto layer 106 b and then heated to fuseit on.

FIG. 19e illustrates an alternative arrangement for joining thesemiconductor core 8 to metal heat sink 111. In this case the joiningoperation is carried out using metal layers 122, 123, instead of usingthe translucent bonding material 112 shown in FIG. 17b. FIGS. 19a- 19 dillustrate manufacturing steps for the arrangement of FIG. 19e. Layers107 a, 110, 106 a, 106 b are grown on the GaAs substrate 105, as shownin FIG. 19a. Then a translucent layer 120 is grown on layer 106 b.Translucent layer 120 may be of SiO₂ having a refractive index of about1.6. One or more through-holes are provided in layer 120 and metallicelectrical contacts 121 are made to layer 106 b through the holes. Areflective metal layer 122 is then formed over translucent surface 120and contacts 121. Layer 122 is thus electrically connected to layer 106b, via contacts 121. Heat sink 111 is coated with a metal layer 123 asshown in FIG. 19b. Coated heat sink 111 is pressed onto layer 122 andthe temperature of the resultant assembly is raised so that one or bothof layers 122, 123 melts, joining the two layers together. Then the GaAslayer 105 is removed, leaving the structure illustrated by FIG. 19c.Then GaP layer 107 b is grown over the structure of FIG. 19c, to providethe structure of FIG. 19d. Then trenches 14, reflectors 16 and p-tracks119 are formed.

GaP layer 107 b can be grown by metalorganic chemical vapour deposition(MOCVD) at a temperature above 500° C., typically 600-700° C. Each ofmetal layers 122, 123, should be solid at the GaP growth temperature.Layer 122 can be of aluminum or silver, or of an alloy. If heat-sink 111is required to be electrically insulated from layer 106, formation ofcontact 121 and its associated hole can be dispensed with. As a furtheroption, reflective layer 122 can be placed directly on the lower surfaceof core 8, dispensing with layer 120, provided layer 122 remainsreflective and layer 106 b remains translucent after growth of layer 107b.

A translucent sheet, such as perforated glass sheet 81 of FIG. 13 may bebonded to the top of the core 8 of the arrangement of FIG. 17e or FIG.19e. Optionally, layer 107 b may be dispensed with in the arrangementsof FIGS. 17e, 19 e, in which case manufacturing stages 17 d, 19 d,respectively are dispensed with.

The various features that have been already described in relation to thechips of FIGS. 2, 5, such as for example the provision elements E or TEwith fuses or series connections, can be incorporated in the AlGaInPlamp arrangements of FIGS. 11, 12, 13, 17 e, 19 e. Each of thesearrangements is capable of high power dissipation in the lightgenerating region (10, 110) of core 8, since there is no semiconductorsubstrate between the AlGaInP layers and the heat sink 62, 86, 111. Thetypical thickness of an ordinary chip substrate is 100-300 microns. Bycomparison, the distance from core 8 to the heat sink (62, 86, 111) canbe as little as a micron. To ensure good heat removal from the AlGaInPlayers it is desirable that the distance between the core and the heatsink 62 be less than 50 microns, and preferably less than 25 micronsand, more preferably still, less than 5 microns. With the improved heatsinking provided by the arrangements of FIGS. 11, 13, 17 e, 19 e thecore temperature during operation will be reduced. Consequently the lampwill have a higher efficiency, since light output of an AlGaInP lampdrops as the junction temperature rises. Also, the rate of degradationof the LED will be reduced and the lamp will last longer.

For each of the arrangements of FIGS. 11, 13, 17 e, 19 e the totalthickness of semiconductor core 8 is preferably less than 50 microns,and it can be 5-10 microns. The depth of trench 14 measured from the topface of semiconductor core 8 to the floor 15 of the trench is preferablygreater than 50% of the thickness of core 8, and more preferably greaterthan 70% of the thickness of core 8. Each of the translucent layersimmediately above and immediately below core 8 (for example 87, 97 inFIG. 13) should be reflective to rays incident to the layer for allangles of incidence greater than 60°, and preferably for all angles ofincidence greater than 55°, and more preferably for all angles ofincidence greater than 45°. To achieve reflection of rays in core 8incident to a translucent member 70, 87, 97, 81, 112, 120 at 55° to themember, the member must have a refractive index that is at least 18%lower than the refractive index of core 8. To achieve reflection of raysin core 8 incident to the translucent member at 45° the member must havea refractive index that is at least 29% lower than the refractive indexof core 8.

Each of the light reflecting layers 70, 86, 87, 97, 111, 112, 120, 122that have been discussed is a non-semiconductor and amorphous. Asemiconductor would not be suitable for any of these layers. It wouldtypically not reflect light incident to it at 60°, 55° or 45°; becauseit would not have a sufficiently low refractive index relative to thatof core 8 to cause the reflection.

Heat sink 62 or 86 or 111 may be of copper-tungsten (CuW), which has acoefficient of thermal expansion (6×10⁻⁶/K) that is close to that ofsemiconductor core 8. The surface of the heat sink adjacent to core 8can in this case be coated with aluminum or silver or some otherreflective material.

The elements E or TE in the AlGaInP embodiments of FIGS. 11, 12, 13, 17e, 19 e can be made small, for example 50 microns wide or less, so thateach element takes very little current. Reducing the current of anelement E or TE reduces lateral voltage drops in the p-type and n-typeAlGaInP layers of the element and enhances uniformity of illumination ofthe element. Each of the elements E or TE can have more than oneconductor (19 or 119) feeding the p-layer (7 b or 107 b).

For each of the arrangements of FIGS. 11, 12, 13, 17 e, 19 e the LEDactive region (10, 110) is preferably less than 50 microns away fromfrom the heat sink, and more preferably less than 25 microns away. Thedepths of light extraction cavities 14 in core 8 can be as great as thethickness of core 8. Connection of tracks 16 to p-layer 6 or 6 b or 106b can be by side contact-tabs, such as 16 t previously described withreference to FIGS. 7a, b.

Referring to the chip construction shown in FIG. 13, layer 81 may be ofmetal instead of glass, in which case layer 86 is made of transparentmaterial for light output downwards. Terminals 83, 84 are insulated fromsheet 81 or eliminated.

For each of the arrangements of FIGS. 11, 12, 13, 17 e, 19 esemiconductor core 8 may be of AlGaAs grown on GaAs, instead of AlGaInPgrown on GaAs. AlGaAs has very nearly the same lattice constant as GaAs,and about the same refractive index as AlGaInP. As another alternative,core 8 may comprise one or more layers of AlGaInP and one or more layersof AlGaAs, the layers having been grown on a GaAs substrate. The use ofAlGaAs layers is less preferable than the use of AlGaInP, because AlGaAslayers give less light output. Window layers 6 b, 7 b, 106 b , 107 b canbe of other semiconductor material than GaP that is translucent to theLED light.

FIGS. 20a, 20 b illustrate an embodiment of the invention for providingwhite light. A chip 200 generating blue-green light of about 487nanometer dominant wavelength is placed over a chip 300 generating amberlight of about 592 nanometer dominant wavelength. The eight triangularchip portions extending beyond the octagonal overlap area of the twochips are arranged by electrical means, or by optical masking, so thatthey do not produce light. Octagonal dotted area 401 represents thelight generated by the two chips. A heat sink, not shown, is providedunder chip 300.

FIG. 21 shows the CIE 1931 (x, y) chromaticity diagram. Point “C”represents a standard white having chromaticity coordinates x=y=0.33.All points within boundary 402 also represent white light. By combiningthe amber 592 nm light (represented by point H1) with the 487 nmblue-green light (represented by point H2) in roughly equal proportionswhite light is produced.

FIG. 22 illustrates layers in the octagonal region 401 of thearrangement of FIGS. 20. Chip 200 comprises a light guide core 208 grownover a sapphire substrate 205. Core 208 comprises p-type and n-typelayers of gallium nitride, or nitride of gallium and other elements suchas indium and/or aluminum; and active region 210 which can be composedof In_(x)Ga_(1−x)N. Active region 210 is arranged for generating the 487nm blue-green light. Chip 200 has several light generating elements E orTE and reflective tracks 216. Chip 200 is basically the same inconstruction as chip 61 of FIG. 9. Chip 300 has a core 308 over a heatsink layer 386. The assembly comprising core 308 and heat sink 386 isthe same as was described for FIG. 14. Active layer 310 of(Al_(x)Ga_(1−x))_(0.5)In_(0.5)P is arranged to generate the 592 nm amberlight. Chips 200, 300 are separated by a thin layer 403 of translucentmaterial, such as epoxy, having a refractive index lower than those ofthe semiconductors. Layer 403 allows light of 60° incidence to layer 403to be guided sideways in each of cores 208, 308. As an example, ifreflector layer 403 has a refractive index of 1.5 and core 208 has arefractive index of 2.5, then reflector 403 will be reflective to raysincident to it from core 208 at an angle of incidence ofsin⁻¹(1.5/2.5)=37°. Thus all 208 rays having an angle of incidence toreflector 403 greater than 37° will in this case be reflected. If themesa 7 b of core 308 has a refractive index of 3.3 then a reflector 403with a refractive index of 1.5 would be reflective to rays from the mesaincident to the reflector at an angle of incidence ofsin⁻¹(1.5/3.3)=27°. Thus in this case all rays from the mesa of core 308having an angle of incidence to reflector 403 greater than 27° will bereflected back into the mesa. Without reflector 403 most of the lightgenerated by active region 210 would be absorbed by semiconductor core308. Cavities 314 can be filled with the same material as layer 403 orwith other material, such as a translucent paste of high thermalconductivity and of refractive index greater than 1.2 and lower than 2.

Amber top light 410 from the LED semiconductor stack 308 is combinedwith blue-green light from LED semiconductor stack 208 to provide outputwhite light 411. Interleaved with the white light 411 are thin bands ofrelatively intense blue-green light 412 from reflectors 216 and bands ofwhitish amber light 413 corresponding to intense amber light emanatingfrom trenches 314. If both chips have many elements E or TE the generaleffect will be that all the light becomes blended and appears white.This is true even if the light is collected and projected by aconvergent lens, providing the lens is defocussed so that it projects ablurred image if the light source.

Active regions 208, 308 are both cooled by heat sink 386. The thicknessof core 308 is preferably less than 40 microns; and more preferably lessthan 15 microns. The thickness can be as little as 5 microns. Thedistance between active region 210 and heat sink 386 is preferably 5-50microns, and more preferably 5-25 microns. It is possible to use for LEDstructure 200 an LED chip such as that described with reference to FIG.3 or FIG. 4. In this case the top of the chip is bonded to atransparent, such as glass, which now becomes a new substrate, and theoriginal substrate 5 is ground down to a small thickness, for example 20microns. The thinned original substrate 5, which is placed overreflector 403, can be mirrored at those portions of its lower surfacethat are below side walls 26, so as not to lose rays 29 (shown in FIG.3) into core 308, The non-mirrored portion of the lower surface ofthinned substrate 5 serves as a window for passing most of the lightfrom core 300.

Part of the top of core 308 may be soldered to part of the under-surfaceof structure 200 to enhance cooling of core 208. In this case each ofthe two mentioned parts is pre-coated with a metal reflector, and thesolder join is between the two metal reflectors. To avoid electricalshorting of core 200 to core 308, the metal reflector on core 308 can beunderlaid with a very thin insulating layer, such as SiO₂.

Structure 200 may act as a support for core 308 during removal of thegrowth substrate of core 308.

Chip 300 can be arranged to generate light of any color in the rangefrom red to yellow. Whichever color is chosen, the corresponding colorwavelength of chip 200 is chosen to be on a line passing through thepoint on the CIE diagram representing the color of chip 300 and thewhite area 402.

To reduce the chance of chip breakage during bonding to chip pads 211,212 (FIG. 20b), sapphire substrate 205 is preferably made thicker than100 microns. Alternatively, it may be 100 microns thick and reinforcedwith a sheet of glass bonded to its top. The heat sink under layer 386(not shown} is perforated or otherwise shaped to allow access toterminals 211, 212 for thermocompression or other electrical bonding.Chips 200, 300 can be shifted slightly one relative to the other insteadof being skewed in order to have access to the chip terminals, in whichcase overlap pattern 401 will be rectangular.

A further embodiment of the invention will now be described withreference to FIG. 23. Light source 420 has LED chips 200, 300 verticallystacked as shown in FIG. 22. Over the top of chip 200 is a layer 421 oftranslucent fluorescent material. In manufacture of chip 300, crystalgrowth of (Al_(x)Ga¹⁻x)_(0.5)In_(0.5)P active region 310 is started withx=0.05 (approximately) for generating red light of about 640 nmwavelength; and as the active region builds up in thickness, the valueof “x” is gradually increased to about 0.35, corresponding to generationof yellow or amber light. At that point growth of the active region 310is stopped. The result of gradually changing the value of “x” is thatall colors in the range from red to yellow or amber are generated byactive region 310, as represented by curve 431 in the spectraldistribution diagram of FIG. 24. Curve 431 has a broad flat top fromabout 590 nm to about 640 nm.

In manufacture of chip 200 for the arrangement of FIG. 23, crystalgrowth of the In_(x)Ga_(1−x)N active region 210 is started with a valuefor “x” suitable for generating blue light of about 470 nm wavelengthand the value of “x” is held constant for part of the growth period ofactive region 210. Then the growth of active region 210 is continued nowgradually increasing the value of “x” to a value that generates greenlight of about 530 nm wavelength, at which point growth of active region210 is terminated. The purpose of this procedure is to provide aspectral distribution for active region 210 represented by curve 432 inFIG. 24. Curve 432 has a blue peak portion 433 corresponding to thegrowth with “x” held constant. The flat portion of curve 432 correspondsto the growth during which “x” is gradually increased.

Translucent fluorescent top layer 421 is arranged to absorb a part ofthe energy at and under peak 433 and to convert the absorbed energy intogreen light represented by dotted curve 434 in FIG. 24. The spectraldistribution of the light 435 emanating from the top of layer isillustrated by curve 436 in FIG. 25. Curve 436 represents a spectraldistribution in the blue (470 nm) to red (640 nm) range that is more orless flat, and similar to the spectral distribution 437 of averagedaylight. Thus there is provided a high power (e.g. 1-50 watts) lightsource giving light similar to daylight and with most of the lightenergy provided by just two LED chips. Only a small proportion of thelight in the spectral distribution 436 is provided by the fluorescentmeans 421.

Fluorescent layer 421 can be arranged for converting UV light, insteadof blue light, to light 434 in FIG. 24. In this case structure 200 isarranged to generate UV light as well as visible light, the visiblelight having a more or less flat spectral distribution from about 450 nmto about 530 nm.

For each of the structures 200, 300 the spectral distribution of thestructure can be broadened by having two or three differing colors forthe elements E, or TE, instead of the spectral distribution beingbroadened by varying “x” during growth of the active region. If theelements E of structure 300 are arranged as a matrix, illustrated inFIG, 26, with columns 471, 472, 473 generating red (R), orange (O) andyellow (Y) light, respectively, then the mixed or integrated light forstructure 300 will have the spectral distribution illustrated by curve481 in FIG, 27. Similarly, If the elements E of structure 200 arearranged as a matrix with columns 471, 472, 473 generating green (G),blue-green (BG) and blue (B) light, respectively, then the average orintegrated light for structure 200 will have the spectral distributionillustrated by curve 491 in FIG, 27. The combined curves 481, 491provide a spectrum distribution that roughly approximates that ofdaylight in the 470-640 nm band.

For each of the structures 200, 300 the spectral distribution of thestructure can be broadened by having two or three differing colors forthe elements E, or TE, instead of the spectral distribution beingbroadened by varying “x” during growth of the active region. If theelements E of structure 300 are arranged as a matrix, illustrated inFIG. 26, with columns 471, 472, 473 generating red (R), orange(O) andyellow (Y) light, respectively, then the mixed or integrated light forstructure 300 will have the spectral distribution illustrated by curve481 in FIG, 27. Similarly, If the elements E of structure 200 arearranged as a matrix with columns 471, 472, 473 generating green (G),blue-green (BG) and blue (B) light, respectively, then the average orintegrated light for structure 200 will have the spectral distributionillustrated by curve 491 in FIG, 27. The combined curves 481, 491provide a spectrum distribution that roughly approximates that ofdaylight in the 470-630 nm range. The differing colors of the elements Eof the structures 200, 300 can be provided by using apertured masks forgrowing the active regions of the elements. Thus,for example, forgrowing the red-generating active regions for columns 471 of chip 300the mask has openings at column positions 471, but not at columnpositions 472, 473.

Dip 485 in FIG. 27 can filled using fluorescent material, over parts ofthe blue columns or as layer 421, that converts blue light to light ofabout 560 nm. To compensate for the loss of blue light caused by thefluorescent material the widths of the blue columns can be increased.

In another embodiment, adapted to provide an alternative arrangement formaking connections to structures 200, 300, the structures are perforatedand joined together at the wafer stage, and then the wafer pair is sawninto separate white-light units (composite chips) 450. A unit 450 isshown in FIG. 28, joined to a major heat sink 460. Perforations 451 inlayers 200, 300 and perforation 461 in heat sink 460 allow access to thefour terminals. As shown, 462, 463 are p, n terminals, respectively, forlight generator 300; and 464, 465 are p, n terminals, respectively, forlight generator 200. This arrangement avoids the triangular overhangs ofFIG. 20.

FIG. 32 illustrates an arrangement according to the invention by whichan electrical connection to the n-layer of structure 200 can be madefrom the top instead of from the bottom. In this case a hole 500 isprovided in sapphire substrate 205 which is through-plated with metal501 to connect n-terminal 465 to an upper n-terminal 565. Line 510 maybe a line along which the wafer is divided into chips, in which case asingle hole 500 can serve the n-terminals of four chips. P-terminals ontop of structure 200 can similarly be provided.

The arrangement of FIG. 20 or FIG. 28 (or FIG. 28 adapted to have topterminals to source 200 as was described in relation to FIG. 32) can beadapted to provide a high power lamp of variable color. In this caselayer 200 is arranged to have alternating blue, 492, and green, 493,columns of elements E shown schematically in FIG. 29. The columns formfinely interleaved luminous stripes when energized. The blue elements Ehave a p-terminal 494 for driving them, and similarly all the greenelements E have a p-terminal 495. Terminal 496 is common to the blue andgreen elements, Layer 300 is arranged to produce red light. Thus thereis provided an RGB lamp in which each of the three colors can beadjusted individually in intensity. The RGB lamp can produce white lightwhen all three colors are turned on simultaneously. The white light inthis case has the spectral distribution 438 shown in FIG. 30. Aconvergent lens may be used to project the light. The projected lightwill have a substantially spatially smooth mix of the three colors,providing the lens is defocussed relative to the interleaved stripes492, 493. The defocussing blurs the blue and green stripe colors intoeach other.

The two-chip light sources of FIGS. 20, 22, 28 can be arranged toprovide red, amber and green light. In this case chip 200 is arrangedfor producing green light and chip 300 is arranged to have a wiringscheme as shown in FIG. 29, with the elements in positions 492fabricated to produce amber light and the elements in positions 493fabricated to produce red light. Such a two-chip light source can beused in a single-housing traffic light; instead of having three separatehousings, one for each of the three colors.

Reflector 403 over core 308 can be of a soft material, such as a rubberysubstance, a gel, a grease, or even a liquid. Small spacers can beattached to the bottom of core 208 that define the thickness of layer403. Cavity filler 314 can be of the same material as reflector 403. Ifreflector material 403 is too soft for laminating wafers it can beapplied to chip pairs instead of to wafer pairs. Using a soft materialto join structures 200, 300 together reduces stresses in the structurescaused by their having different thermal expansion coefficients.

Trenches can be formed in the semiconductor cores using a floatingrotary diamond-tipped saw. The saw perimeter is profiled to provide thedesired shape for the cavity. The floating arrangement can include ashoe that slides on the wafer top surface during cutting of the trench,the shoe being attached to the housing or axle-bearing of the saw, thusensuring that the depth of the cut, measured from the top surface of thewafer, is accurate even if the wafer thickness varies or the wafer iswarped. The machining operations provided by the floating saw can becarried out both for cutting a groove in the semiconductor and forsubsequently shaping metal that is deposited in the groove.

In the various cavity arrangements that have been discussed, the cavitymay have fillets at the junctions of the floors with the side walls, soas to reduce mechanical stresses in the semiconductor. Reflector 316 mayhave on each of its two sides a single reflective surface with a curvedconcave cross-section, instead of two flat surfaces. The same is true ofthe reflector of FIG. 4. The cross-section can have a radius that isabout equal to the height of the reflector.

Various features that have already been described in relation to thechips of FIGS. 2, 5, such as for example the provision of seriesconnections and fuses for the elements E or TE, can be incorporated instructures 200, 300.

What is claimed is:
 1. An LED lamp for generating incoherent visiblelight, comprising: first and second inorganic structures stacked oneover the other and each comprising a lower inorganic semiconductor layerand an upper inorganic semiconductor layer, the lower and uppersemiconductor layers forming a diode and an active region between saidlayers generating light; an imaginary plane generally parallel to saidlayers defining a reference plane; each of said structures as viewednormal to said reference plane having a light generating zone having onouter side perimeter(25) beyond which light generation by the structureceases; wherein said second structure comprises at least onesemiconductor layer that is nitride of one or more of gallium, indiumand aluminum; said first structure generates light of a different colorfrom that generated by said second structure; and said first structureis provided with light-diverting surfaces distant from and within itssaid perimeter as viewed normal to said reference plane, thelight-diverting surfaces being oblique to said reference plane andarranged for diverting light generated in said first structure, thediverted light passing through the said zone of the second structure. 2.An LED lamp according to claim 1 wherein said first structure comprisesat least one layer that is lattice-matched to GaAs.
 3. An LED lampaccording to claim 1 wherein at least part of said first structure islattice-matched to a GaAs substrate that has been removed.
 4. An LEDlamp according to claim 1 wherein at least one of said structurescomprises metallic reflectors that are inclined to said reference planeby an oblique angle.
 5. An LED lamp according to claim 1 includingnon-semiconductor material at said light-diverting surfaces.
 6. An LEDlamp according to claim 1 including a light-passing conductor layer onone of the semiconductor layers of said second structure.
 7. An LED lampaccording to claim 1 wherein said light-diverting surfaces cut theactive region of said first structure.
 8. An LED lamp according to claim1 wherein said diverted light is mixed with light generated by thesecond structure.
 9. An LED lamp according to claim 1 including alight-passing material of lower refractive index than said secondstructure between said first and second structures.
 10. An LED lampaccording to claim 9 wherein said light-passing material forms aninterface with one of said structures that is highly reflective to lightfrom the structure that is incident to the interface at 60 degrees. 11.An LED lamp according to claim 1 wherein at least one of said structuresincludes LED elements that are separated from each other by cavities andwherein a plurality of said LED elements are interconnected for drivingthem in unison.
 12. An LED lamp according to claim 11 wherein each ofsaid cavities contains a metallic reflector.
 13. An LED lamp accordingto claim 1 wherein a plane normal to said reference plane cuts severalof said light-diverting surfaces.
 14. An LED lamp according to claim 1wherein at least one of said structures includes cavities having sidewalls that are oblique to said reference plane and at which lightgenerated in the structure is reflected out of the structure.
 15. An LEDlamp according to claim 1 wherein a member is attached to one of saidstructures, the member having served as a support for the structureduring removal of the growth substrate of one or more semiconductorlayers of the structure.
 16. An LED lamp according to claim 1 wherein atleast one of said structures includes a plurality of cavities eachcontaining metal through which current is passed.
 17. An LED lampaccording to claim 1 including a heat sink for cooling both of saidstructures, the heat sink being joined to one of said structures usingamorphous material and being less than 50 microns away from the activeregion of the other of said structures.
 18. An LED lamp according toclaim 1 wherein one of said second structures includes LED elements andmetal links for feeding currents to the LED elements and wherein forfaulty LED elements that draw more than an allowable amount of currentthe links for feeding the faulty elements are disrupted.
 19. An LED lampaccording to claim 1 wherein one of said structures acts as alight-guide core.
 20. An LED lamp according to claim 1 wherein saidsecond structure is provided with surfaces that divert light generatedby the second structure, the surfaces being distant from and within thesaid boundary of the second structure and inclined to said referenceplane.
 21. An LED lamp according to claim 1 wherein at least one of saidstructures has a hole through it via which an electrical connection ismade to the other of said structures.
 22. An LED lamp according to claim1 wherein one of said structures comprises a plurality of LED elementseach having a top face with an acute angle.
 23. An LED lamp according toclaim 1 wherein said first structure comprises a layer of AlGalnP orA1GaAs.
 24. An LED lamp according to claim 1 wherein one of saidstructures includes LED elements each having an anode and a cathode, thecathode of one of the LED elements being metallically connected to theanode of another of the LED elements.
 25. An LED lamp according to claim1 including soft light-passing material between said first and secondstructures.
 26. An LED lamp for generating incoherent visible light,comprising: first and second inorganic structures stacked one over theother and each comprising a lower inorganic semiconductor layer and anupper inorganic semiconductor layer, the lower and upper semiconductorlayers forming a diode and an active region between said layersgenerating light; an imaginary plane generally parallel to said layersdefining a reference plane; each of said structures as viewed normal tosaid reference plane having a light generating zone having on outer sideperimeter beyond which light generation by the structure ceases; whereinsaid first structure generates light of a different color from thatgenerated by said second structure and wherein said first structure isprovided with light-diverting surfaces distant from and within its saidperimeter as viewed normal to said reference plane, the light-divertingsurfaces being oblique to said reference plane and arranged fordiverting light generated in said first structure, the diverted lightpassing through said second structure.
 27. An LED lamp according toclaim 26 including a light-passing non-semiconductor layer between saidstructures.
 28. An LED lamp according to claim 27 wherein the activeregion of one of said structures has a first portion thereof arrangedfor generating light of a first color and a second portion thereofarranged for generating light of a second color, different from thefirst color.
 29. An LED lamp according to claim 27 wherein said firstand second structures together generate white light.
 30. An LED lampaccording to claim 27 wherein one of structures includes a semiconductorlayer that is lattice-matched to GaAs and the other of the structuresincludes a semiconductor layer that is nitride of at least gallium. 31.An LED lamp according to claim 26 wherein at least one of saidstructures generates light having a spectral distribution with aplateau.
 32. An LED lamp according to claim 26 wherein for at least oneof the structures the structure includes an active layer region having awidth and a thickness and the composition of the active region is variedin the direction of the thickness.
 33. An LED lamp according to claim 26wherein for at least one of the structures the structure includes anactive layer region having a width and a thickness and the compositionof the active region is varied in the direction of the width.
 34. An LEDlamp according to claim 26 including fluorescent material that convertspart of the light generated by one of said structures into light havinga color lying between those produced by said first and secondstructures.
 35. An LED lamp according to claim 26 wherein at least oneof said structures includes an active layer that generates light of afirst color in first portions thereof and light of a second color insecond portions thereof, the first portions being interconnected fordriving them together, and the second portions being interconnected fordriving the second portions together and independently of the firstportions.
 36. An LED lamp according to claim 35 wherein said firstportions are interspersed with said second portions, whereby the lightof said first color can be blended more effectively with that of saidsecond color.
 37. An LED lamp according to claim 26 wherein said firststructure can generate red light and said second LED structure includesan active layer that generates green light in first portions thereof andblue light in second portions thereof, the first portions beinginterconnected for driving them together, and the second portions beinginterconnected for driving the second portions together andindependently of the first portions.
 38. An LED lamp according to claim26 wherein one of said structures is at least part of a chip.
 39. An LEDlamp according to claim 26 wherein said second structure has an undersurface which is provided with a metallic reflector.
 40. An LED lampaccording to claim 26 wherein said first structure is joined to thesecond structure with solder.
 41. An LED lamp according to claim 26wherein one of said structures has trenches that divert light.
 42. AnLED lamp according to claim 26 wherein said light-diverting surfaces cutthe active region of said first structure.
 43. An LED lamp forgenerating incoherent visible light, comprising: a first stack ofinorganic semiconductor layers for generating light of a first color,the first stack having a top light-emitting surface, at least one of thelayers of the first stack being lattice-matched to a gallium arsenidesubstrate that has been removed; a second stack of inorganicsemiconductor layers for generating light of a second color, at leastone of the layers of the second stack comprising nitride of one or moreof gallium, aluminum and indium, and wherein substantially all of thelight passing through said top light-emitting surface passes throughsaid second stack.
 44. An LED lamp according to claim 43 including aconductor layer on one of said stacks that is arranged to belight-passing and through which light generated by said first stackpasses.
 45. An LED lamp according to claim 44 wherein said conductorlayer is of metal and is light-passing by being thin.
 46. An LED lampaccording to claim 45 including a conductor network in electricalcontact with said conductor layer.
 47. An LED lamp according to claim 43including a conductor mesh on one of said stacks.
 48. An LED lampaccording to claim 43 wherein said first stack is provided withlight-diverting surfaces that are inclined to the layers of the firststack and that divert light generated in the first stack, the divertedlight passing through the second stack.
 49. An LED lamp according toclaim 48 wherein said light-diverting surfaces cut the active region ofsaid first structure.
 50. An LED lamp according to claim 43 wherein oneof said stacks has light-extracting trenches.
 51. An LED lamp accordingto claim 43 including a reflector between said stacks, the reflectorbeing highly reflective to light incident thereon at 60 degrees.
 52. AnLED lamp according to claim 51 wherein said reflector is highlylight-passing to light incident thereon at zero degrees, independentlyof the color of the light.
 53. An LED lamp according to claim 43 whereinmost of the light that escapes from said first stack passes through saidsecond stack.
 54. An LED lamp for generating incoherent visible light,comprising: a first stack of inorganic semiconductor layers forgenerating light of a wavelength greater than 550 nanometers, the firststack having a top light-emitting surface; a second stack of inorganicsemiconductor layers for generating light of a wavelength less than 550nanometers, an imaginary plane generally parallel to the layers ofsecond stack defining a reference plane; the second stack as viewednormal to said reference plane having a light generating zone having onouter side perimeter beyond which light generation by the structureceases; a heat sink for cooling both of said stacks; non-semiconductorlight passing material of refractive index greater than one placedbetween said stacks; and wherein substantially all of the light passingthrough said top light-emitting surface passes through said secondstack; said second stack is provided with light-diverting surfacesdistant from and within its said boundary as viewed normal to saidreference plane, the light-diverting surfaces being inclined to thereference plane and arranged for diverting light generated in the secondstack; and said second stack is less than 50 microns away from said heatsink.